1. Field of the Invention
This invention relates to a galvanic cell with high energy and power density, low self discharge, and an open circuit voltage (OCV) which is related to the state of discharge, and to a method of manufacturing such a galvanic cell.
Such a sealed galvanic cell is suitable as an electrical power supply for implantable medical devices with characteristically large current consumptions, such as defibrillators and nerve stimulators. Cells used for implantable devices must be reliable and sealable, self discharge has to be low, and energy density and power density have to be high. Of utmost importance are the discharge characteristics of the cell which should allow a rating of the state of discharge. Precise recognition of the open current voltage just prior to the end of discharge is particularly desired.
2. Description of the Related Art
Prior art power supplies for biomedical applications which allow the determination of the state of discharge under load include those having cathodes comprised of silver compounds, such as argentous chromate, Ag.sub.2 CrO.sub.4, known from U.S. Pat. No. 3,853,627, and those comprised of silver oxide and vanadium oxides known from U.S. Patent Nos. 4,310,609 and 4,391,729. Li/Ag.sub.2 CrO.sub.4 batteries known, for example, from U.S. Pat. No. 3,853,627, are well known as long life power sources for pacemakers. By using only the lowest currents, however, the usable energy of such batteries can be reduced to 60% of the theoretical value. As known from Solar, R. J., "Comparison of accelerated test methods to determine capacity of Lithium Silver Chromate pacemaker batteries", Proc. of the Symp. on Lithium Batteries, Vol. 81-4, 1981, pages 310 to 322, complete reduction of chromium (VI) is undesirably suppressed by the presence of silver metal produced by reduction of silver ions to silver metal. This problem therefore has to be taken into account when silver salts are employed in batteries.
Using Ag.sub.2 CrO.sub.4 or Ag.sub.2 MoO.sub.4, high power batteries with higher energy densities have been built. Solubility of these silver salts in the required organic solvents is too high, however. Therefore, self discharge by direct chemical reaction of CrO.sub.4.sup.- or MoO.sub.4.sup.- with metallic lithium at the anode cannot be lowered below 1 .mu.W/cm.sup.2.
Li/MnO.sub.2 batteries are widely used as high power energy sources. These batteries are sealable and can supply loads of up to several milliamps per square centimeter as known from Lithium Batteries, Gabano, I.P., Ed., Academic Press, 1983, pages 169 to 210. The discharge characteristics of Li/MnO.sub.2 batteries do not reflect the state of charge, however, and the open circuit voltage (OCV) is about 3 V even immediately before the end of discharge (EOD). In German Patent Application No. 2,726,380 (published before examination), a cathode containing CrO.sub.x is described in which x ranges from 2.0 to 2.9. Batteries with such cathodes likewise show a nearly constant discharge voltage until reaching the end of discharge, however. This publication additionally describes additives including PbO.sub.2 or MnO.sub.2 which give the same redox-potential.
U.S. Pat. No. 3,658,592 and German Patent Application No. 2,154,092 (published after examination) describe batteries having cathodes made from metal chromates, such as PbCrO.sub.4. Batteries with low self discharge rates can be built due to the low solubility of PbCrO.sub.4 in organic solvents. These batteries are only suitable for low rate applications (.ltoreq.2 mA/cm.sup.2), however, and have a voltage of about 2 V which is low and which is not sufficient for defibrillators or nerve stimulating devices.
An object of the present invention is therefore to provide an improved cathode material suitable for the above-mentioned kind of batteries which can supply a load of more than 20 mA/cm.sup.2 at a voltage above 1.5 volts, which has a self discharge below 0.1 .mu.W/cm.sup.2, which has cathode constituents selected to provide the desired discharge characteristic, as well as a precise recognition of the end of discharge, such as from a change in slope of the open circuit voltage, and which is sealed.
It is a further object of the present invention to provide a method for producing a novel galvanic cell.
It is yet another object of the present invention to provide a method for supplying electrical power to an implantable device by employing the novel galvanic cell according to the invention.